1. Field of the Invention
The invention relates to methods of towing and installing elongate articles at sea. “Elongate articles” includes for example sections of pipeline, pipeline bundles, tendons for offshore installations and the like. It may also include other extended installations which are limited in their ability to be reeled or folded for transport to their target location. The method can be applied to long or very long articles, (several km) in shallow or ultra-deep water environment, as well as to shorter articles.
2. Description of Prior Art
As one example, many offshore production facilities require large-diameter pipelines for export of hydrocarbons to a processing terminal. The length of such flowlines may be many kilometers. Steel pipe of such diameter cannot be transported in a reeled form for installation at sea. Known techniques for transporting such articles to a subsea location include:
1. Laying the pipe with an S or J lay spread (constructing the pipeline by welding short sections together on a vessel at the target location).
2. Towing shorter lengths of pipe (up to a few km) with known tow techniques and performing surface or bottom connection at the target location to make up the entire flowline section.
A common method of towing is Controlled Depth Tow, where the article is made negatively buoyant with lengths of chain as ballast. These allow the article to rest above the seabed when static, and to rise to a controlled depth when towed at speed. Another option is submerged tow of the article with pencil buoys on surface attached to the pipe with strops.
Each of these methods has significant limitations in the types of installations that can be achieved.